The technique of radiotherapy involves directing a beam of harmful high-energy radiation towards a tumour. The radiation causes damage to the tumour cells which, over time, destroys the cancer. As the beam is harmful, it is necessary to limit the radiation dose that is applied to the healthy tissue, whilst at the same time maintaining the dose delivered to the tumour. Accordingly, some means needs to be provided to de-limit the radiation beam so that its size is no larger than is necessary or achievable. Early radiotherapy machines used a collimation system as shown schematically (along the beam's eye view) in FIG. 1, in which two sets of moving shielding blocks (known as diaphragms) move in mutually perpendicular directions x and y, both axes being perpendicular to the radiation beam (z). Thus, a first pair of blocks 10, 12 move in an x direction to the limit the transverse width of the beam (as viewed in FIG. 1). A second pair of blocks 14, 16 move in the y direction so as to de-limit the width of the beam in that axis. In this way, a beam of any chosen rectangular size up to a maximum achievable size could be used.
Tumours are not generally rectangular, however. As a result, it is now common to use a so-called “multi leaf collimator”, which is made up of individual thin “leaves” of a high atomic number material such as tungsten, each of which can move independently in and out of the beam path in order to block the beam. FIG. 2 shows a generalised multi-leaf collimator which replaces the y collimators 14, 16 of FIG. 1. The x collimators 10 and 12 remain. Thus, the multi-leaf collimator 16 consists of a first bank 18 and a second bank 20, each comprising a large number of thin leaves 22, narrow in the x direction transverse to the beam and relatively long in the y direction transverse to the beam and the z direction parallel to the beam. Their length in the z direction allows sufficient opacity to the x-ray or other beam to achieve an effective shielding effect, and their length in the y direction allows them to be extended into and out of the beam in that direction so as to define any chosen shape.
In some cases, as shown in FIG. 3, the remaining pair of diaphragms 10, 12 are dispensed with altogether, and the leaves are made sufficiently long to shut off the beam completely by overlapping or passing right across the beam as shown in the case of (for example) leaf 24. The join between opposing leaves 24, 26 can either be placed underneath an offset blocking strip 28 (as shown in FIG. 3) or can be achieved by placing the leaves at different points along the z axis so that the two leaves 24, 26 can overlap when viewed in the z direction. This arrangement does, however, mean that the width of the beam in the x direction can only be one of an integer number times the width of the leaves. The arrangement shown in FIG. 2 allows any dimension of a beam width since the x collimators 10, 12 can be moved as desired.
Prior to the development of the MLC, beams were de-limited to the shape of the tumour insofar as existing collimation arrangements permitted. When the multi-leaf collimator became available, novel forms of treatment were made possible such as conformal arc radiotherapy, in which the shape of the beam conforms at all times to the projected shape of the tumour along the instantaneous axis of the beam. This minimises radiation dose to healthy tissue either side of the tumour, and in combination with a rotating source that is able to direct a beam towards the patient from a range of different directions, can result in a very high dose within the tumour and a very small dose outside the tumour.
Conformal arc therapy can, however, only deliver a convex-shaped dose, i.e. one in which the dose steadily decreases away from the dose centre. Further developments in the use of multi-leaf collimators have included techniques such as intensity modulated radiotherapy (IMRT) and other techniques in which more complex shapes created by the multi-leaf collimator allow non-convex dose distributions to be built up over time. Generally, the MLC does not irradiate the entire tumour continuously in such techniques, and otherwise difficult but useful dose shapes can be developed such as a cylindrical dose conforming to the shape of a patient's hip in which (for example) a bone tumour is irradiated leaving the sensitive organs within the hip largely unirradiated. These can result in a need for an off-centre radiation field, as shown schematically in FIG. 4; the radiation field 30 is displaced from the beam's central axis 32, and in order to do this one x collimator 12 is extended across the beam beyond the central axis 32.